Coalesced nanowire structures with interstitial voids and method for manufacturing the same

ABSTRACT

A semiconductor device, such as an LED, includes a plurality of first conductivity type semiconductor nanowire cores located over a support, a continuous second conductivity type semiconductor layer extending over and around the cores, a plurality of interstitial voids located in the second conductivity type semiconductor layer and extending between the cores, and first electrode layer that contacts the second conductivity type semiconductor layer.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to nanowire based structures, inparticular arrays of nanowire light emitting devices.

BACKGROUND OF THE INVENTION

Light emitting diodes (LEDs) are increasingly used for lighting, butstill there are some technological challenges to overcome, in particularwith regard to large-scale processing, in order to reach the realbreakthrough.

Over recent years the interest in nanowire technology has increased. Incomparison with LEDs produced with conventional planar technologynanowire LEDs offer unique properties due to the one-dimensional natureof the nanowires, improved flexibility in materials combinations due toless lattice matching restrictions and opportunities for processing onlarger substrates. Suitable methods for growing semiconductor nanowiresare known in the art and one basic process is nanowire formation onsemiconductor substrates by particle-assisted growth or the so-calledVLS (vapor-liquid-solid) mechanism, which is disclosed in e.g. U.S. Pat.No. 7,335,908. Particle-assisted growth can be achieved by use ofchemical beam epitaxy (CBE), metalorganic chemical vapour deposition(MOCVD), metalorganic vapour phase epitaxy (MOVPE), molecular beamepitaxy (MBE), laser ablation and thermal evaporation methods. However,nanowire growth is not limited to VLS processes, for example WO2007/102781 shows that semiconductor nanowires may be grown onsemiconductor substrates without the use of a particle as a catalyst.One important breakthrough in this field was that methods for growinggroup III-V semiconductor nanowires, and others, on Si-substrates havebeen demonstrated, which is important since it provides a compatibilitywith existing Si processing and expensive III-V substrates can bereplaced by cheaper Si substrates.

One example of a bottom emitting nanowire LED is shown in WO 2010/14032.This nanowire LED comprises an array of semiconductor nanowires grown ona buffer layer of a substrate, such as a GaN buffer layer on a Sisubstrate. Each nanowire comprises an n-type nanowire core enclosed in ap-type shell and a p-electrode with an active layer formed between then-type and p-type regions that form a pn or pin junction. The bufferlayer has the function of being a template for nanowire growth as wellas serving as a current transport layer connecting to the n-typenanowire cores. Further the buffer layer is transparent since the lightthat is generated in the active area is emitted through the bufferlayer.

Although nanowire LEDs have advantageous properties and performance, theprocessing with regard to contacting of the nanowire LEDs requires newroutes as compared to planar technology. Since nanowire LEDs compriselarge arrays of nanowires, thereby forming a three-dimensional surfacewith high aspect ratio structures, deposition of contact material usingline-of-sight processes is a challenging operation.

SUMMARY

One embodiment of the invention is a semiconductor device, such as aLED, includes a plurality of first conductivity type semiconductornanowire cores located over a support, a continuous second conductivitytype semiconductor layer extending over and around the cores, aplurality of interstitial voids located in the second conductivity typesemiconductor layer and extending between the cores, and first electrodelayer that contacts the second conductivity type semiconductor layer andextends into the interstitial voids.

Another embodiment of the invention is a semiconductor device, such as aLED, includes a plurality of first conductivity type semiconductornanowire cores located over a support, a first continuous layer of asecond conductivity type semiconductor extending over and around thecores, a second layer of the second conductivity type semiconductor overthe first layer and comprising a plurality of interstitial voids locatedin this second layer of second conductivity type semiconductor, and afirst electrode layer that contacts the second layer of the secondconductivity type semiconductor and, preferably, does not extend intothe interstitial voids.

A method of making a semiconductor device comprises epitaxially growingplurality of first conductivity type semiconductor nanowire cores fromportions of a semiconductor surface of a support exposed throughopenings in an insulating mask layer on the support, formingsemiconductor active region shells on the cores, growing a continuoussecond conductivity type semiconductor layer extending over and aroundthe cores and the shells, such that a plurality of interstitial voidsare formed in the second conductivity type semiconductor layer extendingbetween the cores during the step of growing, and forming a firstelectrode layer that contacts the second conductivity type semiconductorlayer and extends into the interstitial voids.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described withreference to the accompanying drawings, wherein:

FIGS. 1A and 1B schematically illustrate respective top and side crosssectional view of a basis of a prior art nanowire LED.

FIG. 1C schematically illustrates a side cross sectional view of a priorart nanowire LED structure.

FIG. 2 schematically illustrates a side cross sectional view of anotherprior art nanowire LED structure.

FIGS. 3A-3B schematically illustrate side cross sectional views of theprior art LED of FIG. 2 along lines A and B, respectively in FIG. 3C.FIG. 3C illustrates a top view of the prior art LED of FIG. 2.

FIGS. 4A and 4D schematically illustrate top views of prior art LEDsprior to top electrode deposition.

FIGS. 4B, 4C, 4E and 4F schematically illustrate top views of LEDs priorto top electrode deposition in accordance with alternative embodimentsof the invention.

FIGS. 5A and 5B schematically illustrate side cross sectional views ofthe LED of FIG. 4B after top electrode deposition along lines A and B,respectively in FIG. 5C. FIG. 5C illustrates a top view of the LED ofFIG. 4B after top electrode deposition.

FIGS. 6A and 6B schematically illustrate side cross sectional views ofthe LED of FIG. 4B after top electrode deposition along lines A and B,respectively in FIG. 6C. FIG. 6C illustrates a top view of the LED ofFIG. 4B after top electrode deposition according to another embodimentof the invention.

FIGS. 7A and 7B schematically illustrate side cross sectional views ofthe LED of FIG. 4B after top electrode deposition along lines A and B,respectively in FIG. 5C with a partially air-bridge top electrode.

FIGS. 8A and 8B schematically illustrate side cross sectional views ofan LED with interstitial voids.

FIGS. 9A and 9B schematically illustrate side cross sectional views ofan LED with interstitial voids.

DETAILED DESCRIPTION OF EMBODIMENTS

In the art of nanotechnology, nanowires are usually interpreted asnanostructures having a lateral size (e.g., diameter for cylindricalnanowires or width for pyramidal or hexagonal nanowires) of nano-scaleor nanometer dimensions, whereas its longitudinal size is unconstrained.Such nanostructures are commonly also referred to as nanowhiskers,one-dimensional nano-elements, nanorods, nanotubes, etc. Generally,nanowires with a polygonal cross section are considered to have at leasttwo dimensions each of which are not greater than 300 nm. However, thenanowires can have a diameter or width of up to about 5 μm, for exampleup to 1 μm. The one dimensional nature of the nanowires provides uniquephysical, optical and electronic properties. These properties can forexample be used to form devices utilizing quantum mechanical effects(e.g., using quantum wires) or to form heterostructures ofcompositionally different materials that usually cannot be combined dueto large lattice mismatch. As the term nanowire implies, the onedimensional nature is often associated with an elongated shape. In otherwords, “one dimensional” refers to a width or diameter less than 5microns, such as less than 1 micron, and a length greater than 5microns, such as greater than 1 micron. Since nanowires may have variouscross-sectional shapes, the diameter is intended to refer to theeffective diameter. By effective diameter, it is meant the average ofthe major and minor axis of the cross-section of the structure.

FIGS. 1A and 1B are respective top and side cross sectional views whichschematically illustrate the basis for a nanowire LED structure. Inprinciple, one single nanowire is enough for forming a nanowire LED, butdue to the small size, nanowires are preferably arranged in arrayscomprising thousands of nanowires (i.e., nano-devices or devices) sideby side to form the LED structure. The individual nanowire LED devicesare made up from nanowires 1 having an n-type nanowire core 2 and adiscreet p-type shell volume element 3 at least partly enclosing thenanowire core 2 and an intermediate active region 4 (shown in FIG. 1C)comprising a semiconductor active layer or one or more quantum wells.This forms a light emitting p-i-n junction when the volume element 3directly physically contacts the intrinsic active region 4 or a lightemitting p-n junction if the active region 4 is doped p or n type.However, the nanowire LED devices are not limited to this configuration.For example the nanowire core 2, the active region 4 and the p-typeshell volume element 3 may be made up from a multitude of layers orsegments. As described above, in alternative embodiments, only the core2 and the volume element 3 may form a light emitting p-n junction whenthe volume element 3 directly physically contacts the core 2. The activeregion 4 may be omitted in this case. In order to function as a LED, then-side and p-side of each nanowire 1 has to be contacted. Thus, as usedherein, the core may comprise any suitable nano element having a widthor diameter of less than 5 microns, such as less than 1 micron, and alength greater than 5 microns, such as greater than 1 micron and maycomprise a single structure or a multi-component structure. For example,the core may comprise a semiconductor nanowire of one conductivity typeor it may comprise the semiconductor nanowire of one conductivity typesurrounded by one or more semiconductor shells of the same conductivitytype and the core having a pillar or pyramid shape. For simplicity, asingle component nanowire pillar core will be described below andillustrated in the figures below.

As shown in FIG. 1C, by growing the nanowires 1 on a growth substrate 5,optionally using a growth mask 6 (e.g., a nitride layer, such as siliconnitride dielectric masking layer) to define the position and determinethe bottom interface area of the nanowires 1, the substrate 5 functionsas a carrier for the nanowires 1 that protrude from the substrate 5, atleast during processing. The bottom interface area of the nanowirescomprises the area of the core 2 inside each opening in the maskinglayer 6. The substrate 5 may comprise different materials such as III-Vor II-VI semiconductors, Si, Ge, Al₂O₃ (e.g., sapphire), SiC, Quartz,glass, etc., as discussed in Swedish patent application SE 1050700-2(assigned to GLO AB), which is incorporated by reference herein in itsentirety. In one embodiment, the nanowires 1 are grown directly on thegrowth substrate 5.

Preferably, the substrate 5 is also adapted to function as a currenttransport layer connecting to the n-side of each nanowire 1. This can beaccomplished by having a substrate 5 that comprises a buffer layer 7arranged on the surface of the substrate 5 facing the nanowires 1, asshown in FIG. 2. The buffer layer may be a III-nitride layer, such as aGaN and/or AlGaN buffer layer 7 on a Si substrate 5. The buffer layer 7is usually type matched to the desired nanowire material, and thusfunctions as a growth template in the fabrication process. For an n-typecore 2, the buffer layer 7 is preferably also doped n-type. The bufferlayer 7 may comprise a single layer (e.g., GaN), several sublayers(e.g., GaN and AlGaN) or a graded layer which is graded from high Alcontent AlGaN to a lower Al content AlGaN or GaN. The nanowires cancomprise any semiconductor material, but for nanowire LEDs III-Vsemiconductors such as a III-nitride semiconductor (e.g., GaN, AlInGaN,AlGaN and InGaN, etc.) or other semiconductors (e.g., InP, GaAs) areusually preferred. It should be noted that the nanowire 1 may compriseseveral different materials (e.g., GaN core, InGaN active layer orquantum well(s) and InGaN shell having a different In to Ga ratio thanthe active region). In general the substrate 5 and/or the buffer layer 7are referred to herein as a support or a support layer for thenanowires. Alternatively, a conductive layer (e.g., a mirror ortransparent contact) may be used as a support instead of or in additionto the substrate 5 and/or the buffer layer 7. Thus, the term “supportlayer” or “support” may include any one or more of these elements.

Although the fabrication method described herein preferably utilizesnanowire cores 2 grown on exposed buffer layer 7 portions in openings inthe masking layer 6, as described for example in U.S. Pat. No.7,829,443, to Seifert et al., incorporated herein by reference for theteaching of nanowire fabrication methods, it should be noted that theinvention is not so limited. Nanowire cores 2 grown using VLS methodswith catalyst seed particles or using other methods may be used instead.

In prior art nanowire LEDs 1 shown in FIGS. 1A-1C, the contacting of thep-side 3 of each nanowire 1 is typically accomplished by depositing ap-electrode 8 comprising a conductive layer that encloses the p-typeshell 3 of each nanowire 1 and extends to the insulating layer 6 on thesubstrate 5 or buffer layer 7. The conductive layer of p-electrode 8extends on this insulating layer 6 to adjacent nanowires 1. However,since the nanowires of a nanowire LED are closely spaced (with thespacing between the nanowires 1 shown by line W in FIG. 1A) and being ofhigh aspect ratio, in order to obtain a high luminescence, thep-electrode deposition is a challenging operation. Typicallyline-of-sight processes, such as sputtering or evaporation are used forelectrode deposition. Due to the line-of-side deposition, a preferentialgrowth on the tips of the nanowires and a shadowing effect are observedthat result in a tapering of the p-electrode 8 with decreased thicknesstowards the base of the nanowires 1, as shown in FIG. 1B. Hence, inorder to obtain efficient lateral current spreading, the thickness ofthe p-electrode 8 will become unnecessarily thick on the tips of thenanowires while being insufficiently thick in between the nanowires. Theshadowing effect may also be so severe that there are discontinuities inthe p-electrode. Thus, the p-electrode 8 thickness on the nanowire 1side walls and the bottom surface (e.g., on layer 6) will be verydependent on the length of the nanowires as well as the distance betweenthem. The p-electrode 8 portion on the bottom part of the surface oflayer 6 will be the electric path and will be a grid with widthdepending on the distance between the nanowires, as shown by arrows inFIG. 1A. The current spreading in such a contact layer can be very poorif contacts are too thin in thickness or width, as shown in FIG. 1B.

To overcome the poor current spreading, a planarization of thestructures by growing a continuous p-layer 3 so that the nanowire volumeelements coalesce into a continuous layer can be performed. This willcreate a planar like surface where conventional contacts easily can bedeployed. The continuous p-layer 3 may be grown by the same method(e.g., MOCVD) as the discrete p-shells described in the U.S. Pat. No.7,829,443, except that the growth time is increased to form a continuouslayer instead of discreet shells. By forming a continuous, substantiallyplanar volume element 3 (whose upper surface may not be exactly planardue to curvature of the underlying nanowire and interstitial nanowirespace topography), the contact is moved from being on the sidewalls ofshell volume elements to being only on the top of the continuous volumeelement, as shown in FIG. 2, which is reproduced from an article by“Visible-Color-Tunable Light-Emitting Diodes,” by Young Joon Hong et.al. in the Jun. 3, 2011 on line edition of Advanced Materials. However,this configuration caused relatively high series resistance due tolonger conduction path through thick, resistive p-GaN volume element andlighting up different part of structure due to the thick poorlyconductive p-GaN material.

FIG. 3C illustrates the schematic top view of the coalesced p-volumeelement on nanowire cores layout of Hong et al. FIGS. 3A and 3B areschematic side cross sectional views along lines A and B in FIG. 3Calong the nanowire peaks and sides, respectively. As shown in FIG. 3C,the nanowire cores 2 and active region 4 have a hexagonal crosssectional shape when viewed from the top. The devices 1 with thecontinuous volume element 3 form a tessellation or tiling of the planeshown in FIG. 3C. This means that the continuous p-type layer or volumeelement 3 fills in all of the space between the nanowire cores 2 andactive regions 4 without any overlaps or gaps. Specifically, thehexagonal devices 1 form a regular tessellation of congruent regularhexagons or a “honeycomb tile” where three hexagons come together ateach vertex. In other words, each vertex or “corner” of a hexagoncontacts corners of two other hexagons in the tessellation. As shown inFIGS. 3A and 3B, the current path from the p-electrode 8 to the bottomof the device is shown by line C which extends through most of thevolume element 3 height along the height of the device. Thisconfiguration causes relatively high series resistance due to longerconduction path C through the thick, resistive p-GaN volume element 3.

The present inventor realized that there is a way to get a coalesced,connected, substantially planar p-GaN layer or volume element 3 andenable a shorter conduction path down to the sidewalls of the nanowires1, to reduce the high series resistance of the devices shown in FIGS. 2and 3. Specifically, the continuous p-GaN layer or volume element 3 isgrown such that it contacts plural active regions 4 on plural respectivenanowires cores 2 and such that the volume element 3 contains openingsor interstitial voids between the active regions 4 on respectivenanowire cores 2. The p-type electrode 8 is located on the continuous,substantially planar p-type layer 3 for a lower contact resistance andthe p-type electrode 8 also extends down into the interstitial voids toprovide a shorter conduction path and lower series resistance.

The interstitial voids may be formed using any suitable method. Forexample, the voids may be formed based on how the cores 2 are placedwith respect to each other, which is determined by the lattice geometryin the buffer layer 7 or substrate 5 exposed in openings in theinsulating layer 6. For example, as shown in FIG. 4A, growing thenanowire cores 2 on a (0001) n-GaN buffer layer 7 or on a (111) n-Sisubstrate 5 generates cores 2 with a hexagonal cross-sectional shapewhen viewed from the top. The hexagonal cross sectional shape ispreferably substantially regular hexagonal, i.e., each internal angle ofthe hexagon is about 120 degrees (plus or minus 0-10 degrees due topossible growth irregularities).

When the hexagonal cores 2 are positioned in units cells of three coresat vertices of an imaginary equilateral triangle “T” with vertices ofeach core pointing at two other vertices of adjacent cores, then thetessellated honeycomb structure with no interstitial voids results afterthe active regions 4 and the volume element 3 are formed over the cores2, as shown in FIG. 4A. In contrast, when the cores are with respect toeach other such that one points to less than two other vertices ofadjacent cores, then interstitial voids 9 are formed after the activeregions 4 and the volume element 3 are formed over the cores 2, as shownin FIGS. 4B and 4C.

For example, as shown in FIG. 4B, when the cores 2 are rotated by about30 degrees with respect to the cores 2 shown in FIG. 4A such that eachvertex of a hexagon points at one but not two vertices of one adjacenthexagon, then large triangular interstitial voids 9 are formed in thecontinuous volume element 3 after the active regions 4 and the volumeelement 3 are formed over the cores 2. In another example, as shown inFIG. 4C, when the cores 2 are rotated by less than 90 degrees but morethan 60 degrees such that each vertex of a hexagon points at no verticesof an adjacent hexagon, then small triangular interstitial voids 9 areformed in the continuous volume element 3 after the active regions 4 andthe volume element 3 are formed over the cores 2.

Hexagonal III-nitride based nanowires (e.g., GaN nanowires) always growin the same facet direction based on the crystal orientation of theunderlying material, such as Si (111) or GaN (0001). Thus, specificfacets of the hexagonal cores 2 will always be oriented at the sameangle with respect to the wafer flat of the underlying Si (001)substrate or GaN (0001) buffer layer. Rotating the triangle T in FIGS.4B and 4C with regard to the triangle T in FIG. 4A causes the nanowirecore 2 facets to be rotated in FIGS. 4B and 4C with regard nanowire core2 facets in FIG. 4A. A rotation of the facets of the cores causes thevolume element 3 to be either coalesced when the facets form a honeycombpattern in FIG. 4A or to have voids 9 when the facets do not form thehoneycomb pattern in FIGS. 4B and 4C. In other words, a line normal toeach vertex of a hexagonal core 2 intersects at a point two lines normalto two closest respective vertices of two adjacent cores 2 to form thehoneycomb pattern in FIG. 4A. In contrast, a line normal to each vertexof a hexagonal core 2 does not intersect at a point two lines normal totwo closest respective vertices of two adjacent cores 2 to not form thehoneycomb pattern in FIGS. 4B and 4C. Thus, depending on the orientationof the triangle T with regard to the crystal orientation of theunderlying material, the volume element will be either coalesced asshown in FIG. 4A or contain voids 9 as shown in FIG. 4B or 4C.

In an alternative embodiment, the voids 9 can also be formed using postvolume element 3 growth processing. In this embodiment, a mask (e.g., aphotoresist and/or hard mask) containing openings formed by lithography,such as photolithography, e-beam lithography, nano-imprint lithography,etc. is formed over the coalesced volume element 3. The portions of thevolume element 3 exposed in the mask openings are then etched usinganisotropic etching to form deep holes (e.g., voids 9) in the volumeelement. The etching of the volume element 3 may be done on a volumeelement which is deposited such that it contains no voids 9 (e.g., asshown in FIG. 4A). Alternatively, the etching may be done on a volumeelement 3 which is deposited with narrow voids 9 (e.g., as shown in FIG.4C) to widen the width of the voids 9. In this case, the openings in themask are aligned with the voids 9 before etching. The mask (e.g.,photoresist) is preferably removed after the etching step.

Alternatively, the cores 2 may have a square cross sectional shape (whenviewed from the top) as shown in FIG. 4D, by growing the cores onsubstrates with other crystal orientations. The active regions four(e.g., layers or quantum well(s)) will have substantially the same crosssectional shape (when viewed from the top) as the underlying cores 2.When the square cores 2 are positioned in units cells of four cores atvertices of an imaginary square “S” with vertices of each core pointingat three other vertices of three adjacent cores, then the tessellatedsquare structure with no interstitial voids results after the activeregions 4 and the volume element 3 are formed over the cores 2, as shownin FIG. 4D. In contrast, when the cores are turned with respect to eachother such that each vertex points at less than 3 adjacent vertices(including pointing at not adjacent vertices), then interstitial voids 9are formed after the active regions 4 and the volume element 3 areformed over the cores 2, as shown in FIGS. 4E and 4F.

For example, as shown in FIG. 4E, when the cores 2 are rotated by about45 degrees such that each vertex of a square points at one vertex of oneadjacent square, then large square interstitial voids 9 are formed inthe continuous volume element 3 after the active regions 4 and thevolume element 3 are formed over the cores 2. In another example, asshown in FIG. 4F, when the cores 2 are rotated such that each vertex ofa square points at no vertices of an adjacent square, then smallrectangular interstitial voids 9 are formed in the continuous volumeelement 3 after the active regions 4 and the volume element 3 are formedover the cores 2.

FIG. 5C illustrates the schematic top view of the coalesced p-volumeelement on nanowire cores of FIG. 4B after the p-electrode 8 is formedover the volume element 3. FIGS. 5A and 5B are schematic side crosssectional views along lines A and B in FIG. 5C along the nanowire peaksand sides, respectively. As shown in FIG. 5C, the nanowire cores 2 andactive region 4 have a hexagonal cross sectional shape when viewed fromthe top. The devices 1 with the continuous volume element 3 form anon-tessellated or non-tiled configuration of the plane shown in FIG.5C. This means that the continuous p-type layer or volume element 3 doesnot fill in all of the space between the nanowire cores 2 and activeregions 4, and the triangular interstitial voids 9 toward the insulatingmasking layer 6 are formed.

Specifically, the hexagonal devices 1 do not form a regular tessellationof congruent regular hexagons or a “honeycomb tile” where three hexagonscome together at each vertex. In other words, each vertex or “corner” ofa hexagon does not contact corners of two other hexagons in thetessellation. As shown in FIG. 5A, the volume element 3 fills the entirespace between the cores 2/active regions 4 to layer 6 when viewed incross section along the peaks of the hexagonal cores 2 along line A.Thus, the volume element (i.e., p-GaN layer) 3 is continuous betweenplural cores 2/active regions 4.

However, as shown in FIG. 5B, the volume element 3 does not fill theentire space between the cores 2/active regions 4 when viewed in crosssection along line B located between the peak and edge of the cores 2.This forms the interstitial voids 9 to layer 6 along line B in thevolume element 3. The interstitial voids 9 are partially filled with thep-electrode 8. This provides a shorter current path C2 between thep-electrode 8 and core 2 along line B than current path C1 between thep-electrode 8 and core 2 along line A in FIG. 5A.

Thus, the p-GaN layer 3 has a first portion 3A shown in FIG. 5A whichfills interstitial spaces between the cores 2. The p-GaN layer also hasa second portion 3B shown in FIG. 5B which forms sidewalls of theinterstitial voids 9. The p-electrode 8 contacts the top of the p-GaNlayer 3 (but not the first portion 3A which is not exposed) in FIG. 5Aand contacts the second portion 3B of the p-GaN layer 3 in the voids 9.

Having contacts on the sidewalls of the volume element 3 along somecross sectional lines but not others in the array will reduce theresistive conduction path through the poorly conducting p-GaN volumeelement 8 and will enable the device for more uniform carrier injectionto the active region 4 present on the sidewalls of cores and reducedseries resistance, which provide better LED performance. The p-electrode8 will contact all six sidewalls of each volume element 3 on each core2/active region 4 because there are six voids 9 adjacent to each hexagon(one void per hexagon side) as shown in FIG. 5C.

In an alternative embodiment, the p-electrode 8 fills each entireinterstitial void 9. FIGS. 6A-6C are identical to FIGS. 5A-5C, exceptthat the p-electrode 8 fills the entire interstitial voids 9, as shownin FIG. 6B. The current spreading in the electrode 8 is shown by arrowsin FIG. 6B. This forms a fully connected planar p-electrode 8 on top ofthe device. By depositing a thick enough electrode 8, the voids 9 arefully filled to form a fully connected planar contact or electrode layer8 for improved current spreading. This electrode 8 layer whichcompletely fills the voids 9 as shown in FIG. 6B may be deposited by anon-directional deposition method (e.g., ZnO deposited by atomic layerdeposition), while the electrode 8 layer which does not completely fillthe voids 9 as shown in FIG. 5B may be deposited by a directionaldeposition method (e.g., ITO or Ag deposited by sputtering). If desired,the electrode 8 may be deposited by a combination of non-directional anddirectional deposition methods (e.g., such that the electrode 8 containsa first sublayer deposited by a directional deposition method and asecond sublayer deposited by a non-directional deposition method).

In another embodiment, the electrode 8 may be formed in an air-bridgedconfiguration in the voids 9 as shown in FIGS. 7A and 7B. As usedherein, the term “air-bridged electrode” is taken to mean an electrodestructure that extends between adjacent individual devices to leave anempty space between the adjacent devices. The empty space is preferablysurrounded by the adjacent devices (e.g., by the continuous volumeelement 3 contacting plural active regions 4) on the sides, theair-bridged electrode 8 on the “top” and the support of the devices onthe “bottom”, where the terms top and bottom are relative depending onwhich way the device is positioned. The air-bridged electrode covers thevolume element 8 tips and the voids 9, such that there is an empty space10 beneath the electrode 8 between the nanowire support layer (e.g.,substrate 5, buffer layer 7, insulating mask layer 6, etc.) and theelectrode 8. The air bridged electrode may be formed by providing asacrificial material that fill the bottom of the voids 9, forming theelectrode 8 in the top portions of the voids 9 and then removing thesacrificial material to leave the empty spaces 10 at the bottom of thevoids 9, as taught in U.S. application Ser. No. 13/163,280 filed on Jun.17, 2011 and incorporated herein by reference in its entirety

The voids 9 between the individual devices can not only give an accessto improved electrical contact but also provide photonic crystal effectsfor enhanced light extraction. The photonic crystal configurationsinclude but are not limited to two dimensional hexagonal, triangular,honeycomb or square type that gives a bandgap for or reduces lateralemission. The photonic crystal lattice constant can be given by ½ of thespacing of the nanowire cores 2. The band gap corresponding wavelengthis usually lower than or on the order of the lattice spacing, which isadvantageous in visible light regimes.

While FIGS. 5A-5C and 6A-6C illustrate the formation of the electrode 8in the structure of FIG. 4B, it should be understood that similarstructures will be formed by the formation of electrode 8 in thestructures of FIGS. 4C, 4E and 4F. While the first conductivity type ofthe core is described herein as an n-type semiconductor core and thesecond conductivity type volume element is described herein as a p-typesemiconductor layer, it should be understood that their conductivitytypes may be reversed. The p-type semiconductor layer 3 may comprisesemiconductor materials other than p-GaN, such as p-type InGaN, AlGaN,AlInGaN, etc.

In another aspect of the invention, the devices are suitable to overcomepoor current spreading and to increase light extraction efficiency, by aplanarization of the structures by growing a continuous p-type layer sothat the top surface of the nanowire volume elements coalesce into acontinuous layer, this continuous layer is formed while simultaneouslyincluding voids or holes in the continuous layer that interpose thenanowire volume elements. This creates a planar like surface whereconventional contacts can be deposited and at the same time will createlight-scattering variations in index of refraction within thesemiconductor device, thereby increasing the amount of light which isextracted from the semiconductor device. As used herein, a planar likesurface has a height variation between over two nanowires in the activeLED region the lowest and highest points of 50% or less, such as 0-25%,such as 5-10%. The continuous p-type layer comprising voids may beformed by suitable methods (e.g. MOCVD) as the nanowire volume elements.

In one embodiment, multiple p-type layers are formed around the n-typecores, at least one of the p-type layers is coalesced, and in thiscoalesced layer, the voids are formed. The electrode layer is thenformed on the top of the coalesced p-type layer and the electrode,preferably, does not enter into the voids.

FIGS. 8 and 9 illustrate two schematic cross-sections of embodiments ofnanowire semiconductor devices comprising voids which interpose thenanowire volume elements. In FIG. 8, the process conditions used to formthe second layer of the second conductivity semiconductor are selectedsuch that the growth rate of semiconductor material is highest near theend of the nanowire farthest from the support. In FIG. 8 a, across-section of two nanowires is illustrated part-way into the time ofthe deposition process of the second layer of the second conductivitytype semiconductor. In this embodiment, as depicted in FIG. 8 a, thedevice comprises the nanowire core and active region 12, a firstcontinuous layer of second conductivity type semiconductor 14, and thesecond layer of second conductivity type semiconductor 16. In FIG. 8 b,a cross-section of two nanowires is illustrated later in fabricationprocess after the deposition process has provided enough material suchthat the top surface of the semiconductor device (e.g., top of layer 16)is continuous and a plurality of voids 9 interposes the nanowire volumeelements.

In another embodiment depicted in FIG. 9, the process conditions areselected such that the growth rate of the second conductivity typesemiconductor material is higher at the ends of the nanowire farthestfrom and closest to the support than near the mid-length of thenanowire. In FIG. 9 a, a cross-section of two nanowires is illustratedpart-way into the time of the deposition process of the secondconductivity layer. In this embodiment, as depicted in FIG. 9 b, thedevice comprises the nanowire core and active region 12, the firstcontinuous layer of second conductivity type semiconductor 14, and thesecond layer of second conductivity type semiconductor 16. In FIG. 9 b,a cross-section of two nanowires is illustrated later in the fabricationprocess after the deposition process has provided enough material suchthat the top surface of the semiconductor device (e.g., top of layer 16)is continuous and a plurality of voids 9 interposes the nanowire volumeelements.

As shown in FIGS. 8 b and 9 b, the epitaxial growth of the secondcontinuous layer of the second conductivity type 16 completely enclosesthe interstitial voids 9, such that the voids 9 are completely enclosedby layer 16 on top and sides. In FIG. 8 b, the voids 9 extend to theunderlying layer (e.g., the mask layer 6 or another layer) on the bottomof the voids 9. Thus, in FIG. 8 b, the voids 9 are enclosed on thebottom by this underlying layer. In contrast, in FIG. 9 b, the voids 9are completely enclosed by layer 16 on top, bottom and the sides. Thus,as shown in FIGS. 8 b and 9 b, when the electrode layer 8 is depositedover the p-type semiconductor layer 16, the electrode layer 8 does notextend into the completely enclosed interstitial voids 9.

In a preferred embodiment, the nanowire core 12 comprises n-type GaN,the continuous layer of second conductivity type semiconductor 14comprises p-type AlGaN, and the second layer of second conductivity typesemiconductor 16 (i.e., the layer with the voids 9) comprises p-typeGaN.

In one embodiment, the method for forming the devices shown in FIGS. 8and 9 comprises forming the second layer of a second conductivity type16 in preferred conditions when using MOCVD as the growth method and thesemiconductor material is GaN. In the prior art, wherein no voids orholes interpose the nanowire volume elements, the layer of a secondconductivity type semiconductor is typically formed at a temperature of900° C. or higher in an ambient gas comprising H₂. In this embodiment ofthe present invention, the second layer of a second conductivity typecomprising voids or holes is preferably formed at a lower temperature of900° C. or lower in an ambient gas comprising H₂, or H₂ and N₂, or N₂.For other semiconductor materials, for example InGaN, or AlGaN, thevoids may be formed by other suitable growth conditions. In yet anotherembodiment, layer 16 is a doped semiconductor layer and dopants in thesecond layer (such as Mg, in the case of GaN) may be used to form thevoids or holes.

The nanowire LED structure of the embodiments of the present inventionis either adapted for top emitting, i.e., light emission through thep-electrode, or bottom emitting, i.e., light emission through thesupport layer (i.e., through the conductive layer and/or buffer layerand/or substrate). The electrode contacts may be formed as described inU.S. application Ser. No. 13/163,280 filed on Jun. 17, 2011 andincorporated herein by reference in its entirety. As used herein, theterm light emission includes visible light (e.g., blue or violet light)as well as UV or IR radiation.

For a top emitting device, the p-electrode 8 needs to be transparent(i.e., it should transmit the majority of light emitted by the LED).Indium Tin Oxide (ITO) is a suitable material for the p-electrode, inparticular for the top emitting nanowire LED. The ITO preferably has athickness of 150-900 nm, more preferably 250-650 nm, most preferablyabout 500 nm. Other suitable materials for a p-electrode on a topemitting device are ZnO, doped ZnO and other transparent conductingoxides (TCOs). Important parameters for this material are goodtransparency, electrical conductivity and the ability to make lowresistive contact to the volume element. High thermal conductivity isalso desirable, together with a matching refractive index (depending onconfiguration). In one embodiment of a top emitting nanostructured LEDthe substrate is provided with a reflecting means (e.g., mirror) thatpreferably extends in a plane under the nanowire LEDs. The n-electrodemay be formed on the bottom of the n-Si substrate 5 shown in FIG. 1C.

For a bottom emitting LED, the p-electrode 8 is preferably reflectiveand comprises Ag, Al, etc. The p-electrode may comprise one or moreadditional layers deposited on the p-electrode for improving thereflective and/or conductive properties (e.g., electrode 8 may include atransparent metal oxide, such as ZnO or ITO with an overlying reflectivemirror layer, such as an Ag layer). A separate n-electrode layer, suchas Ti and/or Al is formed in contact with the n-substrate 5 or n-bufferlayer 7 depending on the orientation of the LED device. An addedadvantage of this configuration is a decrease in absorption that pillarsof electrodes incur as compared with side wall contacts.

Although the present invention is described in terms of contacting ofnanowire LEDs, it should be appreciated that other nanowire basedsemiconductor devices, such as field-effect transistors, diodes and, inparticular, devices involving light absorption or light generation, suchas, photodetectors, solar cells, lasers, etc., can be contacted in thesame way, and in particular the air-bridge arrangement can beimplemented on any nanowire structures.

All references to top, bottom, base, lateral, etc are introduced for theeasy of understanding only, and should not be considered as limiting tospecific orientation. Furthermore, the dimensions of the structures inthe drawings are not necessarily to scale.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, on the contrary, it is intended to cover variousmodifications and equivalent arrangements within the scope of theappended claims.

1. A semiconductor device, comprising: a plurality of first conductivitytype semiconductor nanowire cores located over a support; an activeregion shell around each nanowire core; a first continuous layer of asecond conductivity type semiconductor extending over and around thecores; a second continuous layer of the second conductivity typesemiconductor extending over and around the cores; a plurality ofinterstitial voids located within the second continuous layer of thesecond conductivity type semiconductor and extending between the cores;and a first electrode layer that contacts the second continuous layer ofthe second conductivity type semiconductor; wherein: the devicecomprises a light emitting diode (LED) device; the active region shellcomprises at least one quantum well and the first continuous layer ofsecond conductivity type semiconductor contacts the at least one quantumwell to form a light emitting p-n junction at each nanowire coresurrounded by the at least one quantum well shell; the second continuouslayer of the second conductivity type semiconductor extends over andaround the first continuous layer of second conductivity typesemiconductor and the second continuous layer of second conductivitytype semiconductor comprising the plurality of interstitial voidsinterposing the cores; the first electrode layer that contacts thesecond continuous layer of the second conductivity semiconductor doesnot extend into the interstitial voids; the first conductivity typecomprises n-type, the second conductivity type comprises p-type and thefirst electrode layer comprises a p-electrode layer; the supportcomprises an n-type semiconductor buffer layer on a substrate; thesubstrate comprises an n-Si or sapphire substrate, the buffer layercomprises an n-GaN or n-AlGaN layer, the cores comprise n-GaN nanowiresepitaxially extending from portions of the buffer layer surface exposedthrough openings in an insulating mask layer on the buffer layer, the atleast one quantum well comprises an InGaN quantum well, and the secondcontinuous layer of second conductivity type semiconductor comprises ap-GaN layer.
 2. The device of claim 1, further comprising a secondelectrode layer which electrically connects to the n-type nanowirecores.
 3. The device of claim 1, wherein the first electrode layer istransparent.
 4. The device of claim 1, wherein a top portion of thesecond layer of the second conductivity type semiconductor issubstantially planar.